The industry dedicated to energy generation, and particularly the renewable energy industry, requires electric energy conversion systems with increasingly higher powers. To obtain high-power conversion systems, one of the common techniques is to use several conversion units at once, such that for N units a total power equal to N times the power of each conversion unit is obtained. Conversion unit is defined as the electronic device which is responsible for adapting an input electric energy, either alternating current (AC) or direct current (DC), to characteristics imposed on the output thereof, said output being connected to a load or to an energy evacuation point.
The main reason making use of several conversion units at once necessary is the conversion modules. Generally, each conversion unit is formed by a conversion module which is connected to a generator (machine side conversion module) and a conversion module which is connected to an energy grid (grid side conversion module). Each module is formed by power semiconductors such as, insulated gate bipolar transistor (IGBT), Integrated Gate Commutated Thyristor (IGCT) and injection enhanced insulated gate bipolar transistor (IEGT), which break down an input voltage by means of high frequency commutations in order to obtain a voltage the instantaneous mean value of which is the one desired at the output.
Providing the complete systems with several conversion units lead to the conversion units having the necessary uncoupling elements allowing them to be isolated from the other units in the event of failure. When a failure occurs, i.e., when a conversion unit stop being operative (for example, because at least one of its conversion modules fails) and is not capable of generating the output current required therefrom, the maximum power which the system is capable of generating in entirety declines by a factor of 1/N.
However, if the number of conversion units for reducing said factor of 1/N is increased, the complexity of the system as well as its cost and the space necessary for installing it is increased. Likewise, the probability that a failure occurs in any of the conversion units is increased.
Particularly, the requirements of high-power conversion systems with an improved availability have increased considerably due to the boom of the new offshore wind energy generation locations in which the problem due to the difficulty of maintaining them and the great loss of energy involving the lack of availability thereof is increased. Therefore, different application systems and methods have been developed for the purpose of increasing the availability of the conversion systems.
US 2006/0214428 discloses a conversion system provided with a reserve conversion unit which is used when some of the other conversion units are out of service.
This solution therefore involves increasing the number of conversion units of the system which has the clear drawback of involving an increased complexity, cost, and space required. This drawback is made worse in wind turbines where the conversion system is located in the nacelle of the wind generator, where space is really limited due to the high cost of the materials used in enveloping the nacelle.
Additionally, it involves an inefficient use of resources because the reserve unit only acts when some of the other conversion units fail; and it is only capable of compensating the failure of a single conversion unit (if the failure of more units is to be compensated, installing several reserve units is necessary, further worsening the drawbacks described).
In addition, WO2009/027520 takes advantage of the increase which the grid side conversion modules have in the output current, due to the low power factors demanded by the network codes.
To that end it has a conversion system in which all the conversion units are coupled in DC and can work as machine side conversion module or grid side conversion module depending on the status of the different conversion units.
This invention has the drawback that if any of the conversion units is out of service, the conversion system does not have sufficient capacity for meeting the grid requirements for which it was conceived.
Additionally, it forces all the conversion units to be sized for meeting the requirements demanded in the grid side conversion modules, increasing the complexity and the cost thereof, and resulting in an inefficient use of resources when the latter operate as machine side conversion modules.
The object of the present invention is to provide the conversion system with a greater availability with the necessary conversion units for which it has been conceived to operate in nominal conditions.
Therefore, there is the need of an electric energy conversion system with a greater availability and smaller space requirements, capable of being adapted to the failure situations of some of the conversion units forming it, without therefore increasing the number or the performances of said conversion units, and therefore making efficient use of the resources forming it.